[Haskell-cafe] How to best deal with nullPtr from alloca and friends?

[Jason Dagit]

On Tue, Mar 29, 2011 at 2:37 AM, Edward Z. Yang <ezyang at mit.edu> wrote:

> Excerpts from Jason Dagit's message of Tue Mar 29 00:43:10 -0400 2011:
> > I was reading up on the documentation for alloca and friends[1], which
> says,
> > "If any of the allocation functions fails, a value of
> > nullPtr<
> http://www.haskell.org/ghc/docs/7.0.2/html/libraries/base-4.3.1.0/Foreign-Ptr.html#v:nullPtr
> >
> > is
> > produced."
> >
> > It seems like every example of FFI code that I find which uses alloca
> > ignores the possibility of a nullPtr[2, 3, 4].
> >
> > So then I started trying out examples with the help of dmwit and others
> from
> > #haskell.
> >
> > It seems that actually, alloca and friends throw exceptions:
> > dmwit> main = allocaArray (2^30) (\ptr -> print ((nullPtr :: Ptr Double)
> ==
> > ptr))
> > <dmwit> lispy: alloca also throws an exception.
> > <dmwit> lispy: Or rather, allocaBytes throws an exception, and alloca is
> > implemented in terms of allocaBytes, so I'm *guessing* that alloca throws
> an
> > exception.
> >
> > I'm on a 64bit version of windows here with more than 4GB of memory to
> spare
> > for the GHC process. Unfortunately, allocaBytes takes an Int so I can't
> test
> > it with a request larger than the amount of physical ram I have.
>
> You could try testing by setting different limits on the memory usage of
> your
> process with ulimit. I'll give a test, but the prevailing wisdom is that if
> you
> OOM, you're really out of luck.
>

I just want to be clear:  You can get alloca and friends to fail well BEFORE
you are OOM.  Just allocate something larger than the remaining contiguous
space.

If you're allocating a few KB and you run out of memory that can be bad
situation, but I don't think we should treat all allocation failures as if
the sky is falling :)

Jason
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